U4065B Datasheet, PDF (13/34 Pages) TEMIC Semiconductors – FM Receiver

U4065B
With the new U4065B IC, there is a unique access to this problem. This product has an
interference sensor on chip. Thus, an input signal attenuation is only performed if the
interfering signals do generate an intermod signal inside the desired channel. If they do
not, the existing wideband AGC is active but up to 20 dB higher levels. The optimum
AGC state is always generated.
The Figure 20 to Figure 23 on page 14 illustrate the situation. In Figure 20 the AGC
threshold of a standard tuner is high to avoid blocking. But then the intermod signal sup-
presses the desired signal. The interference sensor of the U4065B ensures that the
AGC threshold is kept low as illustrated in Figure 21 on page 14.
In Figure 22 on page 14 the situation is reversed. The AGC threshold of a standard
tuner is kept low to avoid intermod problems. But then blocking makes the desired sig-
nal level drop below the necessary stereo level. In this case, the higher wideband AGC
level of the U4065B enables perfect stereo reception.
By principle, this interference sensor is an element with a third order characteristic. For
input levels of zero, the output level is zero, too. With increasing input level, the output
level is increased with the power of three, thus preferring intermod signals compared to
linear signals. At the same time, a down conversion to the IF level of 10.7 MHz is per-
formed. If a corresponding 10.7 MHz IF filter selects the intermod signals, only an output
is generated, if an intermod signal inside the 10.7 MHz channel is present.
The circuit blocks interference sensor and IF, and detector build up a second IF chain. In
an FM system, the maximum deviation of a 3rd order intermod signal is the triple max
deviation of the desired signal. Therefore, the ceramic IF BPF between pin 11 and pin 9
may be a large bandwidth type. This is all that is needed for this unique feature.
A further narrow band AGC avoids overriding the second IF amplifier. The amplitude
information of the channel is not compressed in order to maintain multipath detection in
the IF part of the receiver.
Figure 20. A High AGC Threshold Causes the Intermod Signal to Suppress the Desired
Signal
Level
Interfering signals
Intermod signal
Intermod signal
Desired
signal
Stereo-level
Noise floor
Desired
frequency
Frequency
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